1. Field of the Invention
The present invention pertains to the transmission of data and communications over cables. More particularly, the present invention pertains to a cable for conducting data signals encoded as analog and digital electrical signals and digital optical signals.
2. Description of the Related Art
In general, the present invention pertains to the art of transmission of electrical and optical signals containing information. The most basic transmission device for transmitting electrical signals is a copper wire, but a single copper wire is subjected to many forms of interference and is therefore generally unsuitable for conducting many types of electrical information, particularly over long distances. One preferred method of reducing interference of electrical signals is the use of a twisted pair of wires which are quite effective in reducing magnetic pickup because the signals induced in successive twists cancel each other. Another cable that is effective in conducting electrical information and which is configured to reduce electrical interference is a coaxial cable. Clad, monomode glass fibers are effective in transmitting optical signal information.
Kao, U.K. Patent No. 1,172,272 discloses a composite coaxial transmission line having an optical fiber core and coaxially arranged metal conductors. Kao discloses that the coaxial metal conductors can be used to transmit electrical power to drive repeaters or other equipment associated with the optical signals. The use of coaxial metal conductors for transmitting power to repeaters was useful before the advent of erbium doped fibers that act as photonic amplifiers when excited by lasers. Because of photonic amplifiers, it is no longer necessary for fiber optic cables to transmit electrical power for their repeaters. It is now possible to construct all-optical networks that require no electronic amplification or switching.
Other previous systems also employ composite cables having optical fibers and coaxial conductors. Winter, et al., U.S. Pat. No. 4,579,420 discloses a cable having coaxially arranged electrical conductors wherein the innermost conductor is hollow and has optical fibers running therethrough. Kumamaru, et al., U.S. Pat. No. 4,097,119, discloses a cable containing both electrical conductors and optical fibers, and reinforced for improved strength in tension and bending. Arnaud, U.S. Pat. No. 4,118,594, discloses a cable system having a coaxial cable primarily for long distance telecommunications and optical fibers for possible future requirements as the telecommunications field seeks to use higher frequency analog and digital signals.
The previous composite systems disclosed cannot meet all present and future demands for transmission of electrical and optical signal information in a single, versatile cable usable over a prolonged service period transcending major technological changes. For example, with respect to present demands, although coaxial conductors are usable in the long distance electrical conduction of telephonic signals, local signals require the use of multiple twisted pairs of wires to accommodate multiple family units and different phone numbers. With respect to future demands, previous composite cable systems provide only a single electrical conductor circuit, limiting the use of the cable systems to a single mode of electrical transmission and foreclosing future multimode electrical transmission possibilities for communications and/or power supply purposes, requiring costly replacement of such cable systems when such future possibilities become realities.